1. Field of the Invention
The present invention relates to a method of bonding a first substrate to a substantially equally sized second substrate. The method is particularly useful for bonding a large size semiconductor wafer having individually complete active devices to a support substrate. The method is also particularly advantageous for bonding a first substrate including at least a thin monocrystalline layer to a transparent substrate and then thinning the first substrate thus improving the optical properties thereof. Further, the method may advantageously be used for production of composites such as active matrix liquid crystal displays or microgap chambers for medical imaging. The invention also includes composites manufactured by the method in accordance with the present invention.
2. Technical Background
Flat panel displays have been extensively used for the screens of lap-top computers and projection devices. The search for an optimal flat panel display continues. For instance, there are continuous efforts to develop panels which are larger in size, lower in energy consumption, more compact, brighter and have higher resolution. Compact, bright, very high resolution panels find application in head-mounted and projection displays. The demand for high resolution liquid crystal displays (LCD) has resulted in efforts to develop an active matrix LCD (AMLCD) with integrated drivers.
Monocrystalline silicon would allow high resolutions with integrated drivers as well as additional functionality on the same chip and would allow standard CMOS processing. The low leakage current of monocrystalline transistors allows minimal transistor size and hence high resolution. Amorphous or polycrystalline silicon layers would be easier to fabricate but they have inferior electronic properties such as low mobility and high leakage currents. These properties limit the reduction in size of the transistors and hence the resolution of the LCD display.
The disadvantage with monocrystalline silicon is its inferior optical transparency. A very thin layer of monocrystalline silicon improves the transparency but its low mechanical strength means that it is preferably supported by a transparent substrate such as glass. Methods of transferring a thin top layer of silicon are known from U.S. Pat. No. 5,206,749 in which a large area semiconductor film is separated from a first substrate and then mounted on a glass substrate. The technique is complex, in particular it is necessary to support the film at all times which requires refined methods of providing and then removing multiple supporting elements. Modifications of this technique are described in U.S. Pat. Nos. 5,256,562 and 5,258,325. In '562 and '325, circuits are formed on a silicon wafer which is diced into tiles. Each tile is separated from the underlying substrate and transferred individually to a common module body and attached using a curing adhesive. The silicon substrate of the transferred tile is etched to reduce its thickness. Hence complex, multi-function circuitry is formed on a common module body using circuit tiles of silicon thin films which are separated from a first substrate, transferred to the common module body and then interconnected and packaged. This procedure is complex and time consuming. However, as only small pieces of film are transferred, the support problems of '749 are partially solved.
Various techniques for bonding a substrate to another substrate such as glass plates are known. Some materials for low temperature bonding have been suggested by D. E.
Booth and C. E. Hunt in the article "Low temperature adhesion bonding methods", Electrochemical Society Proceedings, vol. 95-7, pages 201 to 211. In this article, a method is described for bonding two similar substrates, e.g. microscope slides using various adhesives. The reported chemical tests indicate that among the materials tested curing epoxy bonding films had good survival rate in cleaning, etching and lithography chemicals. No tests on semiconductor substrates are reported, but a speculative application in CCD displays is indicated. It is proposed to illuminate the CCD device from the rear so that the epoxy film is not part of the optical circuit. Because two identical substrates are proposed, e.g. two silicon wafers, the method does not lend itself easily to chemical etching as the etching chemicals would affect both substrates equally. Instead, mechanical thinning of one substrate is proposed which, however, places high loads on the substrate/bonding material interface. Epoxy films have poorer optical properties than two-part epoxy but have about 50% higher overlap shear strength.
It is an object of the present invention to provide a method of fabricating a composite including two substrates and a bonding material between them having improved optical properties.
It is a further object of the invention to provide a method of manufacture of optical devices including two substrates, in which at least one of the substrates is a semiconductor substrate processed by CMOS technology.
It is still a further object of the present invention to provide a method suitable for manufacturing large size composite structures including a semiconductor substrate containing individually completed active devices and circuits which is relatively simple to perform, is economical and is highly reliable.
It is a further object of the invention to provide a transmissive liquid crystal display and a method for producing the same.